The present invention relates to a tooth or bone implant, and more particularly, to a tooth or bone implant having three layers of varying resorbability.
The first attempts to place synthetic materials into the tissues were not made until the end of the eighteenth century, even though the subject has always been of interest to researchers. Those studies did not, however, bring about any satisfactory results. Instead, since the beginning of 1950's there has been varying success in placing titanium or tantalum metal blades into bone tissue, for example the jawbone.
Due to the difficulty of the necessary operation and the risk of infection, the large subperiosteal constructions and the large endosteal blades are becoming increasingly unpopular. These have been replaced by single screws (Br.ang.nemark, Straumann, Bioceram; ref. 1 Albrektson et al.; special reprint. The long term efficacy of currently used dental implants. A review and proposed criteria of success. Int. J. Oral and Max. Fac. Implants. 1. 1986.) and by large conical cylinders resembling a single tooth root. (Frialit; ref. 1.). For the present methods the structure is commonly placed into the alveolar bone. Thus it is possible to achieve a mechanical contact between the bone and the implant. The implant also penetrates the gingiva leaving a protrusion visible. This protrusion acts as a support for the protheses. To prevent bacterial invasion, it is necessary that the epithelium should attach closely to the surface of the protrusion. To achieve this, it is required to always keep the area clean.
The attachment of the implant to the bone in the above methods has often been prevented by an infectious tissue layer. It is possible to avoid this problem by a two stage operation technique in which the implant is totally screwed into the bone during the first stage (ad modum Br.ang.nemark, ref. 1.). After 3-6 months, the second stage is performed during which a protrusion is placed onto the implant. The results have been satisfactory.
Several variations of this method have been introduced (IK-implant, IMZ-implant, Core-vent-implant). The materials used are titanium or Co-Br alloy with Ti by plasma spraying (TiO.sub.2). The healing period is between 3 to 6 months, even though the manufacturer promises a possibility of instant loading after operations. The main disadvantages are the difficulty and the complexity of the method.
Simpler, but less reliable, is the Straumann-method, in which the hollow perforated cylinder protrudes out of the gingiva and is placed into the bone with only one operation. The material is a Co-Cr alloy, which is coated with TiO.sub.2 by plasma spraying. These cylinders were withdrawn from the market in 1986 due to their unreliability.
A third type of implant is the polycrystalline Al.sub.2 O.sub.3 -ceramic, Frialit-implant, which is placed directly after the tooth extraction into the widened root cavity. This implant narrows towards the end with distinct step. Because of the brittleness of the ceramic, these implant can only be used to support a single tooth which is subject to only small bite pressure, such as the front teeth.
The Japanese "Bioceram" Al-crystal-sapphire implant series by Kyoceran are implants that are smaller than the Frialit implant. These implants have a high value for other strength properties, but their ductility is low. Also the annealing has to be done carefully because of the brittleness of the material. The clinical results obtained do not correspond to the results given by the manufacturer. It appears that the bone attachment to Bioceram is not satisfactory. The reason for this is probably the inert nature and the low friction of the surface and the small surface area.
Even a long healing period does not seem to be enough to create a strong attachment.
The minimum dimensions of the bone required for the large and simple attaching mechanisms are a depth of 10 mm and a breadth of 5.5 mm (Straumann). After tooth extraction, the alveolar bone shrinks so much that there is seldom enough left for the placement of such implants.
Both the titanium (Ti) and the Al.sub.2 O.sub.3 implants are too large, because the thin structure is not otherwise able to withstand the necessary mechanical stresses. In addition, the manufacture of Ti is difficult. On the other hand, the small implants (Br.ang.nemark) are complex and expensive, and the smallest, non-metallic (Bioceram) implants are brittle.
The material types used for tooth implants can be classified in the following way:
1. Non-biocompatible Metals, which are non-biocompatible: Co-Cr-alloys, stainless steels, Ni-alloys, noble metals of type IV (Indium, Holmium). PA0 2. Inert materials, which do not react with the tissues but which obtain a close contact with the bone: Al.sub.2 O.sub.3 -ceramics, Ta- and Ti-metal, their alloys (TiV4A16), carbon in its different forms, Teflon. PA0 3. Biactive materials, which attach actively and quickly to the bone i.e. they are surface-reactive and induce bone growth: PA0 1. The implant must be small and strong and fully biocompatible: PA0 2. The implant must have a surface capable of bonding with bone and epithelium and it must have an enhancing effect on the bone growth.
Hydroxyl apatite (HA), calcium phosphate glass (CaP-glass) and CaP-glass ceramics, "bioglasses". PA1 Tricalciumphosphate i.e. TCP (Resorbs i.e. dissolves into the tissue), PA1 thus the elastic Ti and potentially allergic Ni- and Cr-alloys are not viable; PA1 remaining alternatives are sufficiently noble alloys (Au-, Pt-, Pd-alloys) or a hard Ti-alloy. PA1 HA=Hydroxyl apatite PA1 TCP=Tricalcium phosphate PA1 Bioglass or glass ceramic
The composition and the manufacturing method affects the resorption of these materials, and it varies from fully-resorbable to non-resorbable.
The following table represents the materials used in tooth implants and their properties.
______________________________________ 10 N/mm.sup.2 Bone Bonding to Material Strength* bonding epithelium ______________________________________ Al-bioceramic 9-7/7-4 ++ ++ material Ca P-bioceramic 9-0.5/2-0.5 ++ ++ material Hydroxyl apatite 0.5/0.5 +++ + Titanium 99.9 6/5 + + Ti--6 Al--4 V 7/6 .+-. .+-. Ti--6 Al--4 V + 7/6 ++ + bioc. coat. Steels 9.75/7.5 Co-alloys 8.5/7 - - Carbon 5.5 + + Plastic (containing Ca) 1-0.5 .+-. .+-. Noble metal 8.1/5.8- .+-. .+-. 8.8/8.4 Tantalum + + ______________________________________ *tensile strength/fracture stress +++ bonding to bone and to epithelium is very good and furthermore it induces bone growth. ++ bonding to bone is very good + bonding to bone is good .+-. bonding to bone is not well defined - there is no bonding - foreign particle reaction
Current problems associated with biactive materials are as follows:
1. Joining hydroxyl apatite (HA) to metal is problematic. A mechanical joint has been attempted, but, because of the brittleness of HA and it has not led to satisfactory results. Also, plasma spraying has been attempted. However it has not been demonstrated that the crystal structure is conserved, and the surface is transformed to the easily resorbed TCP, so the joint is not durable (World High Tec. Congress, Milano 1986). Also, the coral-based (naturally occurring) HA has not provided any better results than the synthetic one. However, HA is a relatively inexpensive material, onto which collagen fibers of the tissues attach and mineralize very well (Jarcho et al. several studies; ref. 2 de Putter, de Lange, de Groot: Permaucosal dental implants of dense hydroxyl apatite. Fixation in alveolar bone; Abstract: Int. congress on tissue integration in oral and maxillofacial reconstruction, Brussels 1985).
2. CaP-glasses and glass ceramics, i.e. so called bioglasses, have been observed to obtain a good bone bonding (Hencke el al. 1971; ref. 3. Gross et Strung: The interface of various glasses and glass ceramics with a bony implantation bed. J. Biomed. Mat. Res. 251-271: 19, 1985).
The reaction is based on the creation of a SiO.sub.2 -rich layer and on the precipitation of Ca and P. Ca and P then crystallize into HA around the collagen fibers attached to the surface.
An ideal method of implanting a tooth prosthesis would fullfill the following requirements:
Such surfaces are:
At present there are implants that have a metal core in order to obtain a sufficiently small size and high strength. The core is coated with a bioglass (CaP-glass) which is non-resorbable due to its metal oxide content. In addition it has an outer layer, resorbable to bone or biomass, made of CaP-glass (ref. U.S. Pat. No. 4,497,629, Ogino et al.). Thus requirements 1 & 2 are superficially full-filled; however, as the outer layer is resorbed, the inner layer does not achieve a satisfactory bonding to the tissue.